Parents are actual people. The word can be used in the sense of "my parents" - being my mother and father. The word can also be used in the sense of parents as a class of people as in "All parents care about their children". Parental is an adjective associating an action with a parent - Parental care made me the person that I am today. Compare that with - Medical care saved my life. (Medical is an adjective here).

parents is noun while parental is adjective.